In my co-pending application Ser. No. 033,905, filed Apr. 27, 1979 and entitled "Hydrostatic Mold Gate", a mold gate is disclosed which includes a plunger member having an enlarged head slidably mounted adjacent the molding compound inlet conduit. The enlarged head opens and closes the gate responsive to pressure conditions therein. It opens on pressures encountered in the molding inlet conduit to let molding compound into the mold cavity. Then, when the mold cavity is filled and the back pressure increases, the mold gate closes because its mold-cavity facing surface is of larger cross-sectional area than the surface of the plunger opposed to the mold cavity and subject to contact with pressurized molding compound in the conduit.
This surprising result is accomplished because the plunger member has a shaft fitting within a receptacle or sleeve in a manner which is substantially separated from contact with the pressurized molding compound. Accordingly, unbalanced force relationships exist when the elevated pressures on the enlarged head of the plunger from the molding compound inlet and the back pressure from the mold cavity are approximately equal, causing the valve to close.
As described in the cited application, the use of such a hydrostatically operated valve provides significant advantages. For one, it avoids the need for computeroperated adjustment of the gates of a multiple cavity mold which senses in a feedback loop the pressure and temperatures at the control gates, and opens and closes them accordingly. However, for many purposes such a system has been found to have insufficient sensitivity, while the valve gate system of the cited application is directly responsive to the actual pressures themselves, and operates spontaneously without the need for exterior controls. Thus a multiple cavity mold may be provided in which each gate shuts off spontaneously when the mold cavity has been filled, greatly simplifying the requirements for balancing the flow characteristics of a multiple cavity mold.
Other significant advantages are also provided by the hydrostatic type gate of the cited application. For example, it greatly reduces or eliminates the problem of gate freeze, and at the same time provides a gate of larger cross-sectional area to reduce the undesirable effects of material shear and gate stresses, which can actually degrade or decompose molding material due to the kinetic forces applied on the material as it is forced through the gate.
Other problems which can be solved by the hydrostatic mold gate of the cited application relate to the reduction of material suck-back through a gate that stays open too long, or gate drool.
Furthermore, gates of the type described in the cited application can spontaneously reopen during the molding cycle if for any reason the pressure equalization between the cavity and the inlet conduit is merely transient, for example due to shrinkage in the mold or the need for "packing out" the mold. Then, when the pressure in the mold cavity rises again to a predetermined pressure relative to the pressure in the mold cavity, the hydrostatic gate spontaneously recloses again, resulting in a well-finished mold product of high quality and great reproducibility. The valves provide particular advantage in multiple cavity molds since less attention has to be given to balancing the flow characteristics of each cavity in such a mold, because each hydrostatic gate spontaneously shuts each cavity off when the desired pressure conditions have been achieved. Thus, assurance is provided that each cavity in the mold has been properly filled.
By the improvement of this invention, an improved hydrostatic mold gate is provided which is free of sliding parts. In a structure dependent upon the operation of sliding surfaces which operate in contact with hot, pressurized molding compound, it is difficult to seal the sliding surfaces so that molding compound does not seep into the area between the plunger and a receptacle in which it slides. Any molding compound that does seep into that area can interfere with the free sliding of the mold gate, requiring cleaning and repair of the gate.
By this invention, a mold gate of the hydrostatic type is provided in which there are no sliding surfaces available to be interfered with by seeping molding compound. Accordingly, the valve gate operates without the need of maintenance for a greatly increased length of time, while still providing the advantages described above for a hydrostatic type of mold gate.